Photovoltaic (PV) power generation is typically used in a “distributed generation” scheme, in which multiple PV power generators are connected to an electrical grid.
Photovoltaic (PV) power generation has made tremendous strides over the last 10 years in terms of delivered cost of energy. Despite this, the capital costs associated with PV modules and associated items such as inverters and mounting structures remain high and there is a need to reduce these costs further. Therefore, where the cost of energy is fairly high, the ability to capture a high percentage of energy is important. To help PV power generation become more cost competitive and therefore more widely accepted some government agencies, such as the California Energy Commission (CEC) and the federal government have established base financial incentives for this PV system generation (e.g., rebates or subsidies). The California Energy Commission, for example, bases its rebates on PV inverter efficiency. The efficiency used for rebate purposes is a weighted efficiency which seeks to incentivize high efficiencies at the percent of power rating that systems operate at for long periods of time. In other jurisdictions, the same weighted efficiency may be used, but the weighting is different and determined by local solar irradiance conditions. By way of example, the CEC heavily weights operation at 75% power, whereas in Europe 66% power is the more heavily weighted power level.
Within the PV inverter, there are loss mechanisms that are related to and are directly proportional to switching frequency which is determined by a carrier waveform used in pulse-width modulation (PWM) or other switching technique. For this reason, it is desirable to keep the switching frequency as low as possible; however, this must be traded off with the AC waveform distortion which is improved by going to high switching frequencies. Because of these two competing requirements, the selection of switching frequency has always been a compromise and it has traditionally been held constant for a given inverter design. The AC waveform distortion is typically determined by the harmonic content or total harmonic distortion (THD) of the AC waveform. In a perfectly ideal AC waveform, the THD is zero, meaning that there are no harmonic components present on top of the 50 Hz (Europe) or 60 Hz (North America) fundamental component. The standard which sets the maximum allowable THD in North America is IEEE 519. In Europe, there are International Electrotechnique Commission (IEC) standards that govern maximum allowable THD. These standards establish permissible THD limits, although THD limits can also be established from other sources, or can be arbitrarily set.